1. Field of the Invention
The present invention relates to a vehicle steering apparatus of a steer-by-wire system provided with a steering handle operated by a driver for steering a vehicle, a turning actuator for turning steered wheels and a turning control device that drive-controls the turning actuator according to the operation on the steering handle for turning the steered wheels.
2. Description of the Related Art
The development of the steering apparatus of a steer-by-wire system of this type has recently been carried out positively. For example, the following Patent Reference 1 discloses a steering apparatus that detects a steering angle and a vehicle speed, calculates a transmission ratio that decreases with the increase in the steering angle and increases with the increase in the vehicle speed, calculates the turning angle (displacement amount of a rack shaft) of a front wheel by dividing the steering angle by this transmission ratio and turns the front wheel into the calculated turning angle. In this steering apparatus, the calculated turning angle is corrected according to the steering speed that is obtained by time-differentiating the detected handle steering angle, thereby enhancing the steering response and tracking performance of the front wheel. Further, a target yaw rate is calculated by using the detected vehicle speed and the detected handle steering angle, and the calculated turning angle is corrected according to the difference between the calculated target yaw rate and the detected actual yaw rate, thereby realizing the turning control considering the vehicle behavior.
The following Patent Reference 2 discloses a steering apparatus that detects a steering torque and a handle steering angle, calculates two turning angles that increase with the increase in the steering torque and the handle steering angle, and turns the front wheel into the turning angle that is obtained by adding up these calculated turning angles. In this steering apparatus, the vehicle speed is also detected, wherein both turning angles are corrected by this detected vehicle speed, thereby changing the turning characteristic according to the vehicle speed.
[Patent Reference 1]
Japanese Unexamined Patent Application No. 2000-85604
[Patent Reference 2]
Japanese Unexamined Patent Application No. HEI11-124047
The conventional both apparatuses detect the steering angle and steering torque that are operation input values by a driver to a steering handle for steering the vehicle, directly calculates the turning angle of the front wheel by using these detected steering angle and steering torque, and turns the front wheel into the calculated turning angle. However, although the mechanical connection between the steering handle and the steered wheels is unlocked, these turning controls of the front wheel are totally the same in the basic technical concept wherein the turning angle of the front wheel is decided so as to correspond to the steering position and steering force of the steering handle from the viewpoint of a steering method of the front wheel with respect to the operation of the steering handle. The turning angle of the front wheel is not decided so as to correspond to a man's sensory characteristic by these turning methods, so that the driver has a sense of incongruity and has a hard time for driving the vehicle.
Specifically, in the conventional apparatuses, the turning angle that cannot be perceived by the driver is directly decided so as to correspond to the operation of the steering handle, and the vehicle is turned by the turning of the front wheel according to the steering angle. The driver has sensed with a sense of touch or sight the lateral acceleration, yaw rate and turning curvature of the vehicle caused by the turn of the vehicle and has fed them back to the operation of the steering handle for turning the vehicle in a desired manner. In other words, the turning angle of the front wheel with respect to the operation of the steering handle by the driver is a physical quantity that a man cannot perceive. Therefore, the turning angle directly decided with respect to the steering operation by the driver is not decided so as to be adapted to the driver's perception characteristic, with the result that the driver has a sense of incongruity. This makes it difficult for the driver to drive the vehicle.
In the conventional apparatuses, the decided turning angle is corrected according to the difference between the target yaw rate, which is calculated by using the detected vehicle speed and the detected handle steering angle, and the detected actual yaw rate. This is only a correction of the turning angle considering the vehicle behavior, so that the turning angle is not decided according to the yaw rate that the driver is to perceive by the operation of the steering handle. Accordingly, in this case too, the turning angle decided with respect to the steering operation of the driver is not decided so as to be adapted to the driver's perception characteristic, and hence, this makes it difficult for the driver to drive the vehicle.
In order to cope with the aforesaid problem, the present inventors have grappled with the research of a vehicle steering apparatus that can steer a vehicle as adapted to a man's perception characteristic with respect to an operation of a steering handle by a driver. As for the man's perception characteristic described above, the Weber-Fechner's law states that human sensory amount is proportional to the logarithm of a physical quantity of given stimulation. In other words, if the physical quantity of the stimulation given to a human with respect to the operation amount by the human is changed in the manner of exponential function in case where the operation amount is a displacement, or if the aforesaid physical quantity is changed in the manner of exponentiation function in case where the operation amount is a torque, the relationship between the operation amount and the physical quantity can be adapted to the man's perception characteristic. The present inventors have found the following, as a result of applying the Weber-Fechner's law to a vehicle steering operation.
Upon driving a vehicle, the vehicle turns by the operation on a steering handle. The turn of the vehicle changes vehicle motion state quantity such as the lateral acceleration, yaw rate, turning curvature or the like. The driver senses this vehicle motion state quantity with a sense of touch or sight. Therefore, if the vehicle motion state quantity that a driver can perceive is changed in the manner of exponential function or in the manner of exponentiation function with respect to the operation on the steering handle by the driver, he/she can drive the vehicle by operating the steering handle as adapted to the perception characteristic.